Yes, of course, you can unsubscribe anytime you want! If you unsubscribe not later than 24 hours before your Free Trial expires, you won't be charged. If you unsubscribe anytime later, you won't be charged any further after your cancelation date. You can subscribe and unsubscribe as many times as you want.
If you'd like to cancel your subscription please open the app, choose My Profile in the Main Menu, then Settings on the top right corner, then Manage Subscriptions and Manage Subscriptions in Google Play. You will still have access to the classes till the end of your trial period or the month you paid for.